與空白/空值加入MySQL返回沒有結果

Question

我試圖返回一些值（如果它們存在），如果不返回該集合的其餘部分。

SELECT people.first_name, countries1.name AS "Country1" 
FROM addressbook_people AS people 
JOIN root_countries AS countries1 ON people.country1 = countries1.id 

在某些情況下，不會有任何的people.country1提供的值，
但是如果一個它不提供任何結果。

如何重構此查詢，以便在people.country1中沒有值時仍返回people.first_name ？

Answer 1

只是做

SELECT people.first_name, countries1.name AS "Country1" 
FROM addressbook_people AS people 
LEFT JOIN root_countries AS countries1 ON people.country1 = countries1.id 

這將導致久違的root_countries表的相應字段NULL。